The current embedded base of data networks are based on IEEE 802 Local Area Networks, i.e., so-called "Legacy LANs". These Legacy LANs are so-called "connectionless" because network entities exchange packets without establishment of a layer-2 connection. Many existing and emerging applications are designed to run primarily on Legacy LANs. These applications reside on top of so-called "layer-2" and "layer-3" protocols such as Medium Access (MAC) and Internet Protocol (IP), respectively. As is well known in the art, the layers referred to are those of the international standards organization (ISO) seven layer networking reference model.
Asynchronous transfer mode (ATM) with its a) fixed size cell switching, b) sealability from few megabits to hundreds of megabits, c) ability to offer guaranteed quality of service (QoS) on a per connection basis, and d) connection orientation, is viewed as the enabling technology for high-speed multimedia networking. Therefore, it is desired in the art to interconnect Legacy LANs and ATM end-stations, to themselves and to each other, using ATM. This has been achieved in the prior art in a variety of ways.
To describe the existing solutions, consider the example shown in FIG. 1 of two small networks 101 and 103, each representing a different logical subnet at layer-3. LAN 101 has sites 111, 113, and 115 which are interconnected through an ATM wide area network (WAN) 131. Hosts 111-1, 111-2 and 111-3 of site 111 are connected via Ethernet 111-4, hosts 113-1, 113-2, and 113-3 of site 113 are connected via Ethernet 113-4, and ATM hosts 115-1 and 115-2 are connected to an ATM switch 115-6 at site 115. Similarly structured, the second network, LAN 103, has only two hosts per Ethernet site.
A classical method for interconnecting these sites is so-called "bridging and routing". Consider the case of host 111-1 sending a data packet to the MAC address of host 113-2. All stations on Ethernet 111-4, and consequently, bridge 111-5 receives the packet. Bridge 111-5 a) builds broadcast ATM connections to bridge 113-5, hosts 115-1 and 115-2; b) encapsulates host 111-1's data packet on top of the ATM layer, and c) sends it over the ATM connections.
Bridge 113-5, hosts 115-1 and 115-2 receive the information transmitted over the respective ATM connections. Bridge 113-5 strips of the ATM encapsulation, converts the ATM cells into a MAC packet, and sends it to Ethernet 113-4. Thus, all stations on Ethernet 113-4, and consequently, host 113-2 receives the data packet. Hosts 115-1 and 115-2 ignore the received data packet since it is not addressed to them.
lnter-LAN communication according to this technique is achieved through the use of external router 151, since networks 101 and 103 are in different layer-3 subnets. For example, if host 117-1 wants to communicate with host 111-1, host 117-1 sends a data packet to the MAC address of router 151, in which case bridge 117-5 builds an ATM connection to router 151 and sends an ATM encapsulated data packet thereto. Router 151 forwards the packet to bridge 111-5.
A deficiency of this method is that, since it is based on a broadcast principle and thus mimics shared-medium operations, all data packets are broadcast to all ATM destinations, thereby flooding the network with broadcast traffic. Another deficiency is that the broadcast nature of the technique virtually requires a mesh network between all bridges and ATM hosts within a LAN, and all inter-LAN traffic must pass through router 151.
The ATM Forum has developed another bridging solution called LAN Emulation (LANE). LANE relies on a LAN Emulation Server (LES), which performs ATM-to-MAC address resolution, i.e., translation, and a Broadcast and Unknown Server (BUS), which performs data broadcast. The above-described example operates in a LANE environment as follows.
LAN 101 and LAN 103 constitute two different ELANs. As shown in FIG. 2, LAN 101 is served by LES 201 and BUS-203 while LAN 103 is served by LES 211 and BUS 213. As in the previous technique, host 111-1 transmits a data packet with the MAC address of host 113-2. All stations on Ethernet 111-4, and consequently, bridge 111-5, receive, the data packet. Bridge 111-5 either contains within its own information the ATM address of host 113-2, or, if not, it establishes an ATM connection to LES 201 and transmits thereto a so-called "LE.sub.-- ARP.sub.-- request" message to obtain the ATM address of host 113-2. The LE.sub.-- ARP request is defined in the ATM Forum's LAN Emulation Over ATM Specifications, Version 1.0, which is incorporated herein by reference and the contents of which are well known by those skilled in the art.
If LES 201 contains within its own information the requested address, it responds by transmitting it to bridge 111-5. Bridge 111-5 then builds an ATM connection to bridge 113-5, and transmits thereto the data packet. Otherwise, LES 201 broadcasts an LE.sub.-- ARP.sub.-- Request message requesting the ATM address of host 113-2 to all other LAN Emulation Clients (LECs) in ELAN 101, namely bridge 113-5, hosts 115-1 and 115-2. Generally speaking, LAN emulation clients (LEC) are end-stations or bridges that are directly connected to an ATM network. Bridge 113-5 responds to LES 201 with its own ATM address, because it is serving host 113-2, the host whose MAC address has been specified. Here, bridge 113-5 is called a "Proxy LEC", since it represents multiple end-point addresses, e.g., the MAC addresses of hosts 113-1, 113-2, and 113-3. For a more detailed description of the definitions of LEC and proxy LEC reference may be made to the above-mentioned ATM Forum's LAN Emulation Over ATM Specifications, Version 1.0.
Broadcast data packets, such as a so-called "ARP.sub.-- Request", are forwarded to a BUS, which in turn broadcasts them to all LECs. "ARP.sub.-- Requests" are defined in Bell Communications Research (Bellcore) request for comments (RFC) 826, which is incorporated herein by reference. Also, data packets are sent to a BUS until a direct ATM connection is established to the target address within the LAN.
Similar to the previous example, the communications between two Emulated LANs is done via external router 151. Bridge 117-5 receives a data packet from host 117-1 to the MAC address of router 151. Either bridge 117-5 has the ATM address of router 151, or it requests the address from LES 211. After obtaining the ATM address of router 151, bridge 117-5 builds an ATM connection thereto, and transmits the data packet over the connection.
Either router 151 has the ATM address of bridge 111-5, or it requests the address from LES 201. Router 151 then builds an ATM connection to bridge 111-5, and send thereto the data packet. The data packet will be received by bridge 111-5 and passed to hosts 111-1, 111-2 and 111-3. Thus, disadvantageously, all inter-LAN packets must pass through router 151, which may become a communications bottleneck.
A third method builds upon LANE, but incorporates the routing function as well as bridging function into a so called multi-layer LAN switch. Fundamentally, there are three major functions associated with routing: 1) routing, i.e., determination of the layer-3 address of the next-hop-router along the path to the target address, 2) address resolution or translation, i.e. determination of a router's ATM address corresponding to its layer-3 address, and 3) data forwarding, i.e., relaying data packet from one port of the router to another port. A traditional router performs functions (1) and (3) while function (2) is required because an ATM connection must be established between adjacent ATM router hops or to the target ATM address. A multi-layer switch performs only function (3), i.e. data forwarding.
A route server is used to store next-hop router's layer-3 address, and an address resolution protocol (ARP) server is used to resolve, i.e., translate, layer-3 addresses to ATM addresses. Sometimes these functions are merged into one server, a so-called "Route/ARP Server". With a multi-layer LAN switch, the intra-LAN communication is performed just as in LANE using the local LES and BUS of each ELAN. However, inter-LAN communication is different. In the following description, Router 151 of FIG. 2 is assumed to undertake the role of a Route/ARP server.
If host 111-1 wants to talk to host 117-2, then bridge 111-5 acts as a router, obtains the next-hop-routers IP address from the route server 151, and obtains the corresponding ATM address from ARP server 151. It then establishes an ATM connection directly to bridge 117-5 and sends the data packet.
This method is more efficient than using external router since the external router hop is eliminated. Further efficiency is obtained by each multi-layer switch performing fast data forwarding both for layer-2 and 3 packets, while complicated route determination and address resolution functions are logically removed from the switches. Although the Route Server decouples the routing from data forwarding, it only can serve a few logical subnets, and is thus not suited to cover large number of logical subnets. As the number of subnets governed by a single route server increases the efficiencies provided with this approach greatly diminish, since several router hops become practically unavoidable.
Thus, the existing LANE and Route/ARP Server methods work effectively only for small scale local networks. The LANE solution requires one LES and BUS per ELAN, and one ELAN per subnet. If an ELAN becomes large, there will be large numbers of broadcast messages transmitted, which results in a degradation of network performance. Similarly, a single route/ARP Server approach is not suitable for large scale networks, as it also creates performance bottlenecks.
Another problem with the foregoing approaches is the difficulty of handling frequent moves and changes between ELANs when an ELAN may extend beyond corporate boundaries and may span several remote sites. For example, during the life of a project it may be useful for LAN 101 and LAN 103 to form a single LAN. However, with the above solutions, the assigning of hosts to a single LAN is not easily manageable, except in the case of relatively small local networks.